Press machines such as punch presses commonly use, as a press driving source that moves punches forward and backward, a mechanism that rotates a flywheel by means of a rotary electric motor to obtain press driving force using the inertia force of the flywheel, or a hydraulic cylinder. Mechanisms using a flywheel cannot vary a ram speed during strokes. Accordingly, proposals have been made of press machines that use a servo motor instead of the flywheel to vary a punch speed during strokes in order to reduce noise and to improve processing quality.
Where a servo motor is used as a press driving source, it may be difficult to directly obtain a force required for punching. Thus, press machines using a boosting mechanism such as a toggle mechanism have been proposed (for example, the Unexamined Japanese Patent Application Publication (Tokkai-Hei) 8-103897). Attempts have also been made to use a linear motor as a press driving source. Unlike the use of a rotary motor, the use of a linear motor for punch driving eliminates the need for a mechanism that converts rotation into rectilinear motion. This makes it possible to provide a simple structure with a reduced number of parts required.
Press working based on a punch press or the like generally requires the use of the same machine for different machining operations including one needing a greater press tonnage and one needing only a smaller press tonnage. The machining operation needing only a smaller press tonnage generally requires a high speed. Using the same whole press machine for all the operations is contradictory to increased speed and efficiency and saved energy.
Thus, proposals have been made of provision of a second press driving source used for high-speed machining. However, where a boosting mechanism is used, the second press driving source is coupled to an input side of the boosting mechanism, whenever the second press driving source is used, it must be operated via the boosting mechanism. This reduces the efficiency of power transmission. Further, an output side of the boosting mechanism, composed of a toggle mechanism or the like, performs rectilinear reciprocating operations. Consequently, it is difficult to couple the output of the second press driving source, composed of a servo motor or the like, to the output side of the boosting mechanism.